228 ' . ' 



In this study) the varions plant associations have been consldei'ed as 

 members of a succession, and tlacrefovc, in general, transitional. The ulti- 

 raate stage in this region, i .e., the permanent association, is held to be a 

 very mesophytic forest, dominated by Fagus and Acer saccharum. All 

 other plant associations are held to be transitional stages between a plant^ 

 less Condition and this ultimate forest condition. The position of any 

 given plant association in this succession ]ilay be determined accurately 

 only by observation over a long period of time. The successive stages 

 have been worked out carefully in many cases, however, aiid the usual 

 Buccessioli for this region is well known. Two kinds of successions are 

 recognized, namely, biogenic and pllysiogehic. A biogenic succession may 

 be defined as one influenced only by plant and animal life, and therefore 

 Buch a succession will occur only where the physiography is static. lu 

 physiogenic successions, physiographic changes are the controlling factors. 

 In general we have endeavored to determine two points with regard to 

 each plant association, namely, its place in the succession and whether 

 the controlling factors of that succession at that stage are biogenic or phys- 

 iogenic. Lists of species given are usually incomplete but as representative 

 as possible. 



The walls of the gorge and the ravine branching from it are quite 

 favorable for the study of plant successions in such situations. Within 

 the region studied, almost all stages from the bare plantless cliff to the 

 ultimate mesophytic forest can be found. The stage of development of the 

 vegetation on the walls of the gorge seems to be dependent largely upon 

 the length of time that has elapsed since active erosion by the river ceased. 

 The succession is very rapid for a rock cliff. This is explained by the very 

 unstable nature of the rock, the abundance of shale and the favorable con- 

 ditions of rainfall and climate. Very often stages that are usually suc- 

 cessive occur combined, or telescoped, here. Lichens, which usually form 

 the first vegetation on rock cliffs, are absent. No liverworts or ferns' occur. 

 The oaks, which commonly form a stage immediately preceding the ulti- 

 mate forest, seem to be replaced by elms and black locust. Juniperus is 

 the only conifer found. 



The earliest stage of the succession occurs at one point where a cliff 

 occupies the outside of a curve of the stream, and active erosion of its 

 foot is going on. As a result of this condition, the cliff is very steep, even 

 overhanging to a slight extent. The wall is bare of plants, except for algae 



